Of the aromatic compounds used in industry, benzene, toluene and xylenes are of outstanding importance on a volume basis. That mix of compounds, often designated BTX for convenience, is derived primarily from such aromatic naphthas as petroleum reformates and pyrolysis gasolines. The former result from processing petroleum naphthas over a catalyst such as platinum on alumina at temperatures which favor dehydrogenation of naphthenes. Pyrolysis gasolines are liquid products resulting from mild hydrogenation (to convert diolefins to olefins without hydrogenation of aromatic rings) of the naphtha fraction from steam cracking of hydrocarbons to manufacture ethylene, propylene, etc.
Regardless of aromatic naphtha source, it is usual practice to extract the liquid hydrocarbon with a solvent highly selective for aromatics to obtain an aromatic mixture of the benzene and alkylated benzenes present in the aromatic naphtha. That aromatic extract may then be distilled to separate benzene, toluene and C.sub.8 aromatics from higher boiling compounds in the extract. The benzene and toluene are recovered in high purity but the C.sub.8 fraction, containing valuable paraxylene, is a mixture of the three xylene isomers with ethylbenzene. Techniques are known for separating p-xylene by fractional crystallization with isomerization of the other two isomers for recycle in a loop to the p-xylene separation. That operation is hampered by the presence of ethylbenzene.
Concentrated aromatic fractions are also provided by severe cracking over such catalysts as ZSM-5 (U.S. Pat. Nos. 3,756,942 and 3,760,024) and by conversion of methanol over ZSM-5.
Methods available in the art for making alkylation products of aromatic hydrocarbon compounds include the liquid-phase alkylation of aromatics by contact with an alkylating agent in the presence of a crystalline aluminosilicate zeolite which contains various cations including rare earth cations (U.S. Pat. No. 3,251,897). Also, U.S. Pat. Nos. 3,751,504 and 3,751,506 show vapor-phase alkylation of aromatic hydrocarbon compounds by contact of the aromatic hydrocarbon compound with an alkylating agent in the presence of zeolite ZSM-5.